Mixolydian scales are seven-note scales.
Mixolydian is one of the seven Church Modes, or simply modes.
The mixolydian scale conforms to the following scale formula:

scale degree [ formula ]

1

2

3

4

5

6

b7

solfege syllable

do

re

mi

fa

sol

la

te

key of C [ C mixolydian ]

C

D

E

F

G

A

Bb

The mixolydian scale contains all four notes of the seventh chord (1-3-5-b7).
The additional tones are scale degree two, scale-degree four, and scale-degree six.
Scale-degree [b]seven is the most characteristic tone of the mixolydian scale, and uniquely identifies mixolydian.
The basic quality of mixolydian is major.